In recent years, magnetic recording media used in hard disk drives (HDDs) have used a perpendicular magnetic recording method as a technique to realize high densities in magnetic recording. As the layer configuration thereof, for example an underlayer, a magnetic recording layer, a protective film, and a liquid lubricating film are stacked, in order, on a nonmagnetic substrate. In a perpendicular magnetic recording method, there is the advantage that at recorded bits recorded in the perpendicular magnetic recording medium, due to the effect of the demagnetizing field of adjacent recorded bits, the higher the recording density the more stable is the magnitude of the residual magnetization. As a result, a perpendicular magnetic recording medium with excellent thermal stability can be realized.
Various methods for further raising the recording density of perpendicular magnetic recording media have been studied. For example, in a method using what is called a dual-layer perpendicular magnetic recording medium, a medium having a soft magnetic underlayer between the underlayer and the substrate is used. In a dual-layer perpendicular magnetic recording medium, the soft magnetic underlayer steeply draws in the magnetic field generated by the magnetic head, so that the magnetic field gradient is smaller, and the effect of recorded signal spreading is reduced (see for example Non-patent Reference 1 and similar).
In addition, in order to realize still higher recording densities for perpendicular magnetic recording media, finer magnetic crystal grains, improved isolation of such grains, and reduction of the orientation dispersion Δθ50 of the c axis which is the axis of easy magnetization of the magnetic layer, are being studied, and perpendicular magnetic recording media having various layer structures as described below are being proposed.
For example, Patent Reference 1 discloses a perpendicular magnetic recording medium including a soft magnetic underlayer, orientation controlling underlayer the main components of which are NiCr and NiCu and having the fcc structure, and a recording layer comprising a perpendicular magnetization film having the hcp structure. Further, Patent Reference 1 discloses that at least one element from among Fe, Al, Rh, Pd, Ag, Pt and Au is added to the orientation controlling underlayer; and that a nonmagnetic amorphous layer of which the main component is one element among Ta, W and Mo is used on the side below the orientation controlling underlayer; and that an orientation controlling intermediate layer comprising either an Ru alloy or Ti is used between the orientation controlling underlayer and the recording layer.
Further, Patent Reference 2 discloses a magnetic recording medium having at least a underlayer, orientation control layer, magnetic recording layer and protective layer on a substrate, wherein the orientation control layer has a stacked structure having at least a seed layer and an intermediate layer, the seed layer is disposed on the substrate side of the intermediate layer, and moreover the seed layer has a Cu—Ti alloy layer the main component of which is Cu. It is further disclosed that the intermediate layer has as the main material at least one among Ru, Re, and an alloy of these, and has the hcp structure. It is further disclosed that by providing two layers in the intermediate layer, both crystal orientation and finer structure are attained.
Patent Reference 3 discloses a perpendicular magnetic recording medium having, in order on a nonmagnetic substrate, an underlayer, a magnetic recording layer, a protective layer, and a liquid lubricating layer, and which is characterized in that the underlayer includes nonmagnetic NiFeCr. It is also disclosed that an intermediate layer including one among the nonmagnetic materials CoCr, CoCrB, Ru, and Pd is included between the underlayer and the magnetic recording layer.
Patent Reference 4 discloses a perpendicular magnetic recording medium having, in order on a nonmagnetic substrate, a soft magnetic underlayer, an intermediate layer, a magnetic layer which is a CoCr based alloy layer, a protective layer, and a liquid lubricating layer, and in which the magnetic layer comprises a granular-structure first magnetic layer and a nongranular-structure second magnetic layer. It is further disclosed that the intermediate layer comprises an alloy including at least one of metal from among Ti, Re, Ru and Os having the hcp structure.
Patent Reference 5 discloses a perpendicular magnetic recording medium comprising, stacked in order on a nonmagnetic substrate, a seed layer, an underlayer, a magnetic recording layer and a covering layer, wherein the magnetic recording layer comprises a structure in which are stacked magnetic layers of different materials. It is further disclosed that as the seed layer, a NiFe alloy or an alloy of NiFe with at least one among B, Si, Nb and Mo added, or Co or an alloy of Co with at least one among B, Si, Nb, Mo, Fe and Ni added, is used, and that as the underlayer, for example a metal with the hcp structure such as Ti, Zr, Ru, Zn, Tc, Re or similar, or a metal with the fcc structure such as Cu, Rh, Pd, Ag, Ir, Pt, Au, Ni and Co, or similar is used.
Patent Reference 6 discloses a perpendicular magnetic recording medium comprising, stacked in order on a nonmagnetic substrate, a soft magnetic underlayer, an intermediate layer, a magnetic layer which is a CoCr based alloy layer, a protective layer, and a liquid lubricating layer, and wherein the magnetic layer comprises a granular-structure first magnetic layer and a nongranular-structure second magnetic layer. It is also disclosed that the intermediate layer comprises one metal among Ti, Re, Ru and Os having the hcp structure or an alloy including at least one metal among Ti, Re, Ru and Os.
Patent Reference 7 discloses a magnetic recording medium having, on a substrate, a soft magnetic underlayer, FeCoB seed layer, fcc structure crystal orientation controlling layer, nonmagnetic underlayer, and magnetic recording layer, wherein the crystal orientation controlling layer is formed from an alloy including at least one element from among Ni, Fe, Co, Cu, Rh, Ir, Pd, Pt, Al, Au and Ag. Patent Reference 8 discloses a method of manufacture of a perpendicular magnetic recording medium comprising, stacked in order on a nonmagnetic substrate, at least an underlayer, a magnetic recording layer, a protective layer and a liquid lubricant layer, wherein a seed layer which is an NiFe or other Ni based alloy is further provided below the underlayer comprising Ru or an alloy including at least Ru such as RuW, RuTi, RuAl, RuCu, RuSi, RuC, RuB, RuCoCr, or similar. It is further disclosed that the magnetic recording layer comprises a structure called a granular layer in which fine particles of a CoCrPt alloy have an oxide such as SiO2 or TiO2 segregated at the grain boundaries.
On the other hand, Patent Reference 9 discloses a perpendicular magnetic recording medium including a seed layer including amorphous Ta, a nonmagnetic fcc alloy underlayer including Ni and W, a nonmagnetic hcp underlayer and a magnetic layer, and states that an RuCr30 underlayer is used as the hcp underlayer, and also discloses that it is desirable that noble metal elements such as Ru and Pt be removed insofar as possible from within the medium.    Patent Reference 1: Japanese Patent Application Laid-open No. 2008-34060    Patent Reference 2: Japanese Patent Application Laid-open No. 2010-44842    Patent Reference 3: Japanese Patent Application Laid-open No. 2002-358617    Patent Reference 4: Japanese Patent Application Laid-open No. 2003-168207    Patent Reference 5: Japanese Patent Application Laid-open No. 2005-196898    Patent Reference 6: Japanese Patent Application Laid-open No. 2006-277950    Patent Reference 7: Japanese Patent Application Laid-open No. 2008-84413    Patent Reference 8: Japanese Patent Publication No. 4224804    Patent Reference 9: Japanese Translation of PCT Application No. 2010-518536    Non-patent Reference 1: “Suichoku Jiki Kiroku no Saishin Gijutsu” (The latest technology of perpendicular magnetic recording), H. Nakamura, ed., CMC Publishing (2007), pp. 127-131
In this way, numerous techniques have been proposed for raising the recording densities of perpendicular magnetic recording media. However, the characteristics of a perpendicular magnetic recording medium depend on the components and composition of each stacked layer, and on the stacking order and other matters, and in the techniques proposed in the prior art it cannot be said that these matters are all optimized, and hence medium characteristics have merits and demerits. Therefore in recent years there have been demands for further improvements to the characteristics of perpendicular magnetic recording media.
In order to attain a high signal-noise ratio (SN ratio) through increases in the signal output and decreases in the noise of perpendicular magnetic recording media, orientation dispersion in the magnetic recording layer must be made as small as possible. Further, in order to reduce noise in magnetic recording media, crystal grain diameters in the magnetic recording layer must be decreased.
On the other hand, the seed layer or intermediate layer has the function of controlling the crystallinity, orientation, crystal grain diameters and similar of the magnetic recording layer formed thereabove, and are known to affect the characteristics of the magnetic recording layer. Hence in order to decrease crystal grain diameters of the magnetic recording layer material, decreasing the crystal grain diameters in the seed layer or intermediate layer is effective.
However, when the film thickness of the seed layer or intermediate layer is reduced, a decline in the crystal orientation of the magnetic recording layer material and inhibition of magnetic isolation of magnetic crystal grains occur, and it is also known that the magnetic characteristics of the magnetic recording layer decline. Hence in consideration of these matters, it is necessary to control the film thickness of the seed layer or intermediate layer while maintaining or improving the magnetic characteristics of the magnetic recording layer.